Class II major histocompatibility complex molecules are heterodimeric transmembrane proteins expressed on B lymphocytes, monocytes and other antigen presenting cells. One function of Ia molecules is to present foreign peptide antigens to CD4+ T cells. In addition, MHC class II antigens also function as receptors to regulate B lymphocyte and monocyte proliferation and differentiation. These functions may be mediated by the coupling of Ia molecules to intracytoplasmic signaling pathways. B cell lymphomas transfected with class II genes have been used to study the function of MHC molecules. Transfectants which express class II Abeta chains lacking their cytoplasmic domains have a defect in antigen presentation to T hybridomas, perhaps due to a failure to induce the cotimulatory molecule B7. They are also impaired in intracytoplasmic signaling. Both of these defects can be corrected by treatment with cAMP analogues, suggesting that the cytoplasmic tail is associated with a cAMP-mediated signal transduction pathway. The preliminary experiments, we have identified the amino acids in the Abeta cytoplasmic domain which are required for effective antigen presentation. We propose to further define the function of the cytoplasmic domain in antigen presentation, signal transduction, and the differentiation of antigen presenting cells. We will utilize different biological systems in each of our four specific aims. In Aim I, we will use B lymphoma transfectants to define the mechanisms through which Abeta molecules with cytoplasmic domain mutations are associated with an antigen presentation defect. We will examine the induction of B7 and the contribution of other costimulatory molecules. In Aim 2, we will characterize the requirement for the cytoplasmic tail in class 11-mediated signal transduction in B lymphoma transfectants. We will focus on cAMP and tyrosine phosphorylation- mediated pathways and their interaction with downstream functional events. In Aim 3, we will use a sarcoma cell line, SaI, to determine the requirement for the Abeta cytoplasmic domain in a non-professional antigen presenting cell. SaI cells will be transfected with the genes for class Il molecules containing mutations in the Abeta cytoplasmic domain. These transfectants will then be used to identify the amino acids of the cytoplasmic domain required for in vivo tumor rejection. These sequence requirements will be compared to those mediating antigen presentation to T hybridomas. In addition, the signal transduction pathways associated with class II molecules in non-professional antigen presenting cells will be analyzed. in Aim 4, we will utilize a transgenic mouse in which an Abeta transgene with a truncated cytoplasmic domain has been introduced into a class 11-deficient animal. We will examine the role of the Abeta tail in the differentiation of B cells and monocytes and the requirement for cAMP and tyrosine phosphorylation transduction pathways in these functions.